From 60e1775b874015a3451e4bde10a8eb30701b1165 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Tue, 6 Jul 2021 09:44:36 -0700
Subject: Initial commit.

---
 sim/decode.sv | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 sim/decode.sv

(limited to 'sim/decode.sv')

diff --git a/sim/decode.sv b/sim/decode.sv
new file mode 100644
index 0000000..5c9a9c9
--- /dev/null
+++ b/sim/decode.sv
@@ -0,0 +1,82 @@
+module decode
+    #(  parameter   UROM        = "<no file specified>"
+    ,   parameter   UIP_BITS    = 15
+    ,   parameter   UROM_BITS   = 8
+    ,   parameter   BUS_BITS    = 16
+    ,   parameter   OPCODE_BITS = 8
+    )
+    (   input   bit                 clk
+    ,   input   bit                 reset
+    ,   input   bit [UIP_BITS-1:0]  uip
+    ,   inout   bit [BUS_BITS-1:0]  abus
+    ,   inout   bit [BUS_BITS-1:0]  dbus
+    );
+
+bit [OPCODE_BITS-1:0] opcode;
+bit [BUS_BITS-1:0] a;
+bit [BUS_BITS-1:0] b;
+bit needmore;
+
+bit [UIP_BITS-1:0] new_uip;
+assign new_uip = {opcode, {(UIP_BITS-OPCODE_BITS){1'b0}}};
+
+typedef enum
+    { CLEAR
+    , DECODE
+    , OUTADDR
+    , OUTADDR_SEL0
+    , OUTADDR_SEL1
+    , OUTDATA
+    , OUTDATA_SEL0
+    , OUTDATA_SEL1
+    } CtrlBit;
+
+bit [UROM_BITS-1:0] ctrl;
+urom#(UROM, UIP_BITS, UROM_BITS) urom(uip, ctrl);
+
+bit [1:0] asel;
+assign asel = {ctrl[OUTADDR_SEL1], ctrl[OUTADDR_SEL0]};
+
+bit [1:0] dsel;
+assign dsel = {ctrl[OUTDATA_SEL1], ctrl[OUTDATA_SEL0]};
+
+bit [BUS_BITS-1:0] aout;
+assign aout =
+    (asel == 0) ? a :
+    (asel == 1) ? b :
+    (asel == 2) ? {{(BUS_BITS-UIP_BITS){1'b0}}, new_uip} :
+    {(BUS_BITS){1'bX}};
+
+assign abus = ctrl[OUTADDR] ? aout : {(BUS_BITS){1'bZ}};
+
+bit [BUS_BITS-1:0] dout;
+assign dout =
+    (dsel == 0) ? a :
+    (dsel == 1) ? b :
+    (dsel == 2) ? {{(BUS_BITS-1){1'b0}}, needmore} :
+    {(BUS_BITS){1'bX}};
+
+assign dbus = ctrl[OUTDATA] ? dout : {(BUS_BITS){1'bZ}};
+
+always @(posedge clk) begin
+    if (reset || ctrl[CLEAR]) begin
+        opcode <= 0;
+        a <= 0;
+        b <= 0;
+        needmore <= 1;
+    end else if (ctrl[DECODE]) begin
+        automatic bit m = dbus[7];
+        automatic bit i = dbus[6];
+        automatic bit [1:0] field = dbus[5:4];
+        automatic bit [3:0] shift = dbus[3:0];
+        needmore <= m;
+        if (field == 0) begin
+            opcode <= {i ? ~opcode[OPCODE_BITS-1-4:0] : opcode[OPCODE_BITS-1-4:0], shift};
+        end else if (field == 2) begin
+            a <= {i ? ~a[BUS_BITS-1-4:0] : a[BUS_BITS-1-4:0], shift};
+        end else if (field == 3) begin
+            b <= {i ? ~b[BUS_BITS-1-4:0] : b[BUS_BITS-1-4:0], shift};
+        end
+    end
+end
+endmodule
-- 
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